Handover execution and communication resumption in wireless access system

ABSTRACT

Handover execution and communication resumption in a wireless access system is provided. Performing a handover includes communicating with a serving base station through a first communication link and receiving from the serving base station, handover information associated with at least one candidate target base station. The method also includes transmitting handover indicator to the serving base station indicating a handover operation to a selected target base station and performing a ranging procedure with the selected target base station to establish a second communication link with the selected target base station while maintaining the first communication link with the serving base station. The method also includes being released from the serving base station in response to a handover complete status message from the selected target base station to the serving base station, or resuming normal communication with the serving base station if the handover fails with the selected target base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(a), this application claims the benefit ofearlier filing date and right of priority to Korean Application No.10-2004-0042642, filed on Jun. 10, 2004, the contents of which arehereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to a wireless access system, andmore particularly to handover execution in a wireless access system.

BACKGROUND OF THE INVENTION

The IEEE802.16e system is part of a global standardization for broadbandwireless access systems. IEEE802.16e does not have a hierarchicalstructure of HLR, VLR, MSC, BSC, RNC, etc. unlike second and thirdgeneration mobile communications systems, but does include a mobilesubscriber station (MSS), a base station (BS), and an authenticationservice authorization server (ASA server). A physical (PHY) layer and amedium access control (MAC) layer are defined between the BS and theMSS.

A handover process in a broadband wireless access system according to arelated art is explained as follows.

1. Pre-Processing for Handover

Pre-processing for handover includes broadcasting associated informationfrom a base station to a neighboring base station to inform all mobilesubscriber stations within a respective cell of the associatedinformation (Network Topology Advertisement). The pre-processing alsoincludes measuring a channel quality of the neighboring base stationbased on the associated information (MSS Scanning of Neighboring BS).The pre-processing also includes matching parameters between the basestation and the neighboring base station, such as for example, aninitial mobile subscriber station power, a time difference forsynchronization, and other parameters (Association Procedures).

a. Network Topology Advertisement

In network topology advertisement, the base station broadcastsinformation associated with network configuration to all mobilesubscriber stations within a cell via a MOB_NBR-ADV MAC message, therebyproviding all mobile subscriber stations within the cell with theinformation pertaining to neighboring base stations.

MSS Scanning of Neighboring BS

In MSS scanning of neighboring BS, a mobile subscriber station (MSS)scans a neighboring base station (BS) and requests a scanning intervalfor scanning the neighboring BS from the BS via a MOB_SCN-REQ MACmessage. The BS then transmits a MOB_SCN-RSP MAC message in response tothe request to assign an interval for scanning (scanning interval) theneighboring BS. Alternatively, the BS may directly transmit theMOB_SCN_RSP MAC message without a request from the MSS (unsolicitedresponse). In the case of an unsolicited response, the scanning intervaland an offset unit for scanning start are assigned on a frame unitbasis.

c. Association Procedures

During association procedures, the MSS performs a ranging process with aBS to normally join a cell. In the ranging process, the MSS acquiresbasic information for communications from the BS. In other words, theMSS performs BS scanning to select a new BS and then performs theassociation procedures. The MSS and the BS transmit a RNG_REQ MACmessage and a RNG_RSP MAC message, respectively, to set values such asfor example, a power offset value, a timing offset value, and othervalues of the MSS to suitable values. Transmission of the RNG_REQ MACmessage is called an initial ranging operation and is a basic operationthat the MSS may perform in a network entry procedure. When a handoveris performed, a target BS accepts a new MSS and transmits itemsassociated with the respective cell of the new MSS to the former servingBS and stores information pertaining to the MSS.

2. Handover Process

In a handover (HO) process, the MSS initiates handover with theneighboring BS based on the channel quality information acquired in thehandover pre-processing.

a. Cell Selection

In a cell selection procedure, a cell is changed to enable the MSS tomake a new registration to a BS, allowing for reception of a signalhaving an SINR (signal to interference noise ratio) superior to that ofa signal transmitted from the BS of the current cell, prior to a normalregistration of the MSS to a cell. In such case, since the MSS does notperform the registration procedure, the BS is unable to determine amoving situation of the MSS.

b. HO (handover) Initiation

In HO initiation, a handover may be initiated by the MSS or the BS. Whenthe BS requests (initiates) a handover, the BS transmits a MOB_BSHO-REQMAC message to the MSS. On the contrary, when the MSS requests ahandover, the MSS transmits a MOB_MSSHO-REQ MAC message to the BS.

If the MSS transmits the MOB_MSSHO-REQ MAC message, the MSSpreferentially transmits the SINR of a signal received from aneighboring BS to the BS to enable the MOB_MSSHO-REQ MAC message to betransmitted to a current serving BS. The current serving BS then selectsa candidate BS as a target BS for performing the handover.

In a case where the BS receives the MOB_MSSHO-REQ MAC message from theMSS or in a case when the BS has not yet transmitted the MOB_BSHO-REQMAC message to handover the MSS, the BS may allow the handover of theMSS after confirmation of an acknowledgement (ACK) for handing over aspecific MSS from the neighboring BS. After receiving theMOB_BSHO/MSSHO-REQ MAC message, the MSS/BS transmits aMOB_MSSHO/BSHO-RSP MAC message to inform the BS/MSS of a target BS forthe handover.

c. HO Cancellation

In HO cancellation, after the MSS/BS transmits the MOB_MSSHO/BSHO-REQMAC message, the MSS may cancel the handover. In canceling the HO, theMSS sets a specific field (HO_Type=01) of a MOB_HO-IND MAC message andtransmits the set field to the BS. The BS receives the message andcancels the proceeding handover.

d. Termination with the Serving BS

In termination with the serving BS, the MSS transmits the MOB_HO-IND MACmessage to inform the serving BS that the handover is normally completedand then completes the handover operation. In completing the handoveroperation, the MSS sets a specific field (HO_Type=00) of the MOB_HO-INDMAC message to transmit the information pertaining to the normalhandover termination to the serving BS. The serving BS receives theMOB_HO-IND MAC message from the MSS and terminates a MAC state machineallocated to the MSS. The serving BS also terminates an ARQ (automaticrepeat request) connection and all connections associated with datatransmission.

e. HO Rejection

In HO rejection, the MSS is capable of rejecting a handover recommendedby the BS. In rejecting a handover, the MSS sets a specific field(HO_Type=10) of the MOB_HO-IND MAC message and transmits the message tothe BS. The BS receives the rejection message from the MSS, re-seeks fora target BS and re-transmits the MOB_BSHO-RSP MAC message to the MSS.

FIG. 1 is a diagram illustrating a handover process requested by amobile subscriber station (MSS).

Referring to FIG. 1, an MSS 11 performs a scanning procedure to measurequalities of signals transmitted from neighboring BSs (S110). If the MSS11 determines that a handover needs to be performed based on results ofthe neighboring BS scanning procedure, the MSS 11 transmits a handoverrequest message (MOB-MSS-HO-REQ) to a serving BS 12 (S111).

The serving BS 12 receives the handover request message and transmits ahandover pre-notification message (HO-pre-notification) includinginformation that the MSS 11 has requested a handover to a neighboringBS, such as for example, handover target BSs (target BSs) 13 and 14(S112, S113). The handover pre-notification message includes anidentifier (MSS identifier) of the MSS 11 requesting the handover,connection parameters, capabilities, a required bandwidth, and qualityof service information.

Each of the neighboring handover target BSs 13 and 14 receives thehandover pre-notification message and transmits a handoverpre-notification response message (HO-pre-notification-response) to theserving BS 12 in response to the handover pre-notification message(S114, S115). The handover pre-notification response message may includeacknowledgement information pertaining to the handover pre-notificationmessage and capability information pertaining to the requested qualityof service.

The serving BS 12 selects one of the neighboring handover target BSs 13and 14 according to the response messages for the handoverpre-notification and transmits a handover confirm message (HO-confirm)to the selected target BS 13 or 14 (S116). The serving BS 12 thentransmits a handover response message (MOB-BSHO-RSP), includinginformation pertaining to the selected handover target BS, to the MSS 11(S117). The MSS II receives the handover response message and transmitsa handover indicator message (MOB-HO-IND) to release a connection fromthe MSS 11 to the serving BS 12 (S118). Upon receipt of the handoverindicator message from the MSS 11, the serving BS 12 releases itsconnection to the MSS 11 (S119).

The MSS 11 then executes a fast ranging (Fast Ranging_IE), a rangingrequest (RNG-REQ), and a ranging response (RNG-RSP) with the target BS14 (S120, S121, S122). The target BS 14 becomes the serving BS 14 andthe MSS 11 completes an initial network entry execution after handover(S 123).

FIG. 2 is a diagram illustrating a handover process requested by a basestation (BS).

Referring to FIG. 2, a serving BS 22 triggers a procedure(trigger-to-release-MSS) to release a connection from an MSS 21according to a corresponding decision (S210).

The serving BS 22 then transmits a handover pre-notification message(HO-pre-notification) including information required for the handover ofthe MSS 21 to neighboring handover target BSs 23 and 24 (S211, S212).The handover pre-notification message includes an identifier (MSSidentifier) of the MSS 21 requesting the handover, connectionparameters, capabilities, a required bandwidth, and quality of serviceinformation.

Each of the neighboring handover target BSs 23 and 24 receive thehandover pre-notification message and transmit a handoverpre-notification response message (HO-pre-notification-response) to theserving BS 22 in response to the handover pre-notification message(S213, S214). The handover pre-notification response message may includeacknowledgement information pertaining to the handover pre-notificationmessage and capability information pertaining to the requested qualityof service.

The serving BS 22 then transmits a handover request message to the MSS21 based on the information received via the handover pre-notificationresponse message (S215). The handover request message includesinformation pertaining to the target BSs 23 and 24 available for thehandover. The MSS 21 then performs a neighboring BS scanning procedureto measure quality of signals received by the neighboring target BSs, asrecognized via a handover response message (S216). In a case where thetarget BS 24 is found for the handover is found as a result of theneighboring BS scanning, the MSS 21 transmits a handover responsemessage including target BS information for the handover to the servingBS 22 (S217). The serving BS 22 then transmits a handover confirmmessage to the target BS 24 to which the MSS 21 intends to perform thehandover (S218).

The MSS 21 then transmits a handover indicator message (MOB-HO-IND) forreleasing a connection from the MSS 21 to the serving BS 22 (S219). Uponreceipt of the handover indicator message, the serving BS 22 releasesits connection to the MSS 21 (S220).

The MSS 21 then executes a fast ranging (Fast Ranging_IE), a rangingrequest (RNG-REQ), and a ranging response (RNG-RSP) (S221, S222, S223)with the serving target BS 24 for the handover and then completes aninitial network entry execution after handover (S224).

FIG. 3 is a diagram illustrating a process by which an MSS re-enters aformer serving BS, such as in the case of a call drop.

Referring to FIG. 3, an MSS 31 performs a scanning procedure to measurequalities of signals transmitted from neighboring BSs (S310). If the MSSdetermines that a handover needs to be performed based on results of theneighboring BS scanning procedure, the MSS 31 transmits a handoverrequest message (MOB-MSS-HO-REQ) to a serving BS 32 (S311).

The serving BS 32 receives the handover request message and transmits ahandover pre-notification message (HO-pre-notification) includinginformation that the MSS 31 has requested a handover to a neighboringBS, such as for example, handover target BSs 33 and 34 (S312, S313). Thehandover pre-notification message includes an identifier (MSSidentifier) of the MSS requesting the handover, connection parameters,capabilities, a required bandwidth, and quality of service information.

Each of the neighboring handover target BSs 33 and 34 receive thehandover pre-notification message and transmit a handoverpre-notification response message (HO-pre-notification-response) to theserving BS 32 in response to the handover pre-notification message(S314, S315). The handover pre-notification response message may includeacknowledgement information pertaining to the handover pre-notificationmessage and capability information pertaining to the requested qualityof service.

The serving BS 32 selects one of the neighboring handover target BSs 33and 34 according to the response messages for the handoverpre-notification and transmits a handover confirm message (HO-confirm)to the selected target BS 33 or 34 (S316). The serving BS 32 thentransmits a handover response message (MOB-BSHO-RSP) includinginformation pertaining to the target BS for the handover to the MSS 31(S317). The MSS 31 receives the handover response message from theserving BS 32 and transmits a handover indicator message (MOB-HO-IND) torelease a connection from the MSS 31 to the serving BS 32 (S318). Uponreceipt of the handover indicator message, the serving BS 32 releasesits connection to the MSS 31 (S319).

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to handover execution andcommunication resumption in a wireless access system that substantiallyobviates one or more problems due to limitations and disadvantages ofthe related art.

An object of the present invention is to provide a method of efficientlyresuming communications in a wireless access system when a mobilesubscriber station (MSS) returns to a former serving base station (BS)in case of a handover failure.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, inone embodiment, a method of performing handover operation in a wirelesscommunication system includes communicating with a serving base stationthrough a first communication link and receiving from the serving basestation, handover information associated with at least one candidatetarget base station, wherein the handover information comprises at leastquality of service capable of being handled by the at least onecandidate target base station. The method also includes transmitting ahandover indicator to the serving base station indicating a handoveroperation to a selected target base station, wherein the mobilesubscriber station determines whether to perform handover in response tothe handover information, and performing a ranging procedure with theselected target base station to establish a second communication linkwith the selected target base station while maintaining the firstcommunication link with the serving base station, wherein the rangingprocedure comprises attempting to establish an uplink synchronizationand power adjustment between the mobile subscriber station and theselected target base station. The method also includes being releasedfrom the serving base station in response to a handover complete statusmessage from the selected target base station to the serving basestation, or resuming normal communication with the serving base stationif the handover fails with the selected target base station.

The method may also preferably include receiving a handover responsefrom a serving base station upon the serving base station transmitting ahandover confirmation to a handover target base station, wherein thehandover response comprises at least quality of service informationrequested to be handled by the handover target base station. The methodmay also preferably include, in a mobile subscriber station initiatedhandover, transmitting a handover request to a serving base stationthrough a first communication link prior to receiving the handoverresponse from the serving base station. The method may also preferablyinclude, in a service base station initiated handover, receiving ahandover request from the serving base station prior to transmitting ahandover response to the serving base station.

The mobile subscriber station may preferably be released from theserving base station when the handover complete status message indicatessuccessful handover. The mobile subscriber station may preferably resumenormal communication with the serving base station when the handovercomplete status message indicates handover failure. The step ofperforming the ranging procedure may preferably include receiving aframe slot allocation from the target base station for a fast ranging,transmitting a ranging request to the target base station using theallotted frame slot, and receiving a ranging response from the targetbase station.

The handover complete status message from the target base station to theserving base station may preferably be an inter-base station backbonemessage. The handover indicator may preferably include one of (1)serving base station release, (2) handover cancel, and (3) handoverrejection.

In another embodiment, a method of performing handover operation in awireless communication system includes receiving a handover request froma mobile subscriber station through a first communication link,transmitting a handover pre-notification to a target base station, andtransmitting a handover response upon the serving base station receivinga handover confirmation from the target base station. The method alsoincludes receiving a handover indicator from the mobile subscriberstation indicating a handover operation to the target base station andperforming a setup procedure between the mobile subscriber station andthe target base station to establish a second communication link withthe target base station while maintaining the first communication linkwith the serving base station. The method also includes releasing themobile subscriber station or continuing normal communication with themobile subscriber station in response to a handover complete statusmessage from the target base station to the serving base station.

In yet another embodiment, a method of performing handover operation ina wireless communication system comprises transmitting a handoverpre-notification to a target base station. The method also comprisestransmitting a handover response upon the serving base station receivinga handover confirmation from the target base station and transmitting ahandover request to a mobile subscriber station that is in communicationwith the serving base station through a first communication link. Themethod also comprises receiving a handover response from the mobilesubscriber station indicating a handover operation to the target basestation. The method also comprises performing a setup procedure betweenthe mobile subscriber station and the target base station to establish asecond communication link with the target base station while maintainingthe first communication link with the serving base station. The methodalso comprises releasing the mobile subscriber station or continuingnormal communication with the mobile subscriber station in response to ahandover complete status message from the target base station to theserving base station.

The handover request may preferably comprise a possible handover targetbase station. The method may also preferably comprise receiving ahandover complete message from the target base station to the servingbase station.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings. It is to be understood that both the foregoinggeneral description and the following detailed description of thepresent invention are exemplary and explanatory and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a diagram illustrating a handover process requested by amobile subscriber station (MSS).

FIG. 2 is a diagram illustrating a handover process requested by a basestation (BS).

FIG. 3 is a diagram illustrating a process by which an MSS re-enters aformer serving BS, such as in the case of a call drop.

FIG. 4 is a diagram illustrating a handover process initiated by a MSS,according to one embodiment of the present invention.

FIG. 5 is a diagram illustrating a handover process initiated by a BS,according to one embodiment of the present invention.

FIG. 6 is a diagram illustrating a process by which an MSS resumescommunications with a former serving BS, in case of a handover failure,according to one embodiment the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 4 is a diagram illustrating a handover process initiated by amobile subscriber station (MSS), according to one embodiment of thepresent invention. Referring to FIG. 4, an MSS 41 performs a scanningprocedure to measure qualities of signals transmitted from neighboringbase stations (BSs) (S410). If the MSS 41 determines that a handoverneeds to be performed based on results of the neighboring BS scanningprocedure, the MSS 41 may transmit a handover request message (e.g.,MOB-MSS-HO-REQ) to a serving BS 42 (S411).

The serving BS 42 receives the handover request message and may transmita handover pre-notification message (e.g., HO-pre-notification) to aneighboring BS, such as for example, handover target BSs (target BSs) 43and 44 (S412, S413). The handover pre-notification message maypreferably include information that the MSS 41 has requested a handover,as well as an identifier (MSS identifier) of the MSS requesting thehandover, connection parameters, capabilities, a required bandwidth, andquality of service information.

Each of the neighboring handover target BSs 43 and 44 may receive thehandover pre-notification message and may transmit a handoverpre-notification response message (e.g., HO-pre-notification-response)to the serving BS 42 in response to the handover pre-notificationmessage (S414, S415). The handover pre-notification response message maypreferably include acknowledgement information pertaining to thehandover pre-notification message and capability information pertainingto the requested quality of service.

The serving BS 42 may select one of the neighboring handover target BSs43 and 44 based on the handover pre-notification response message andmay transmit a handover confirm message (e.g., HO-confirm) to theselected target BS 43 or 44 (S416). The serving BS 42 may transmit ahandover response message (e.g., MOB-BSHO-RSP), including informationpertaining to the possible handover target BS, to the MSS 41 (S417). TheMSS 41 receives the handover response message and may transmit ahandover indicator message (e.g., MOB-HO-IND) to release a connectionfrom the MSS 41 to the serving BS 42 (S418).

An exemplary configuration of the handover indicator message is shown inTable 1.

TABLE 1 Syntax Size Notes MOB_HO_IND_Message Format( ) { ManagementMessage  8 bits Type = 56 reserved  6 bits Reserved; shall be set tozero HO_IND_type  2 bits 00: Serving BS release 01: HO cancel 10: HOrejection 11: Resume Communication Target_BS_ID 48 bits Applicable onlywhen HO_IND_type is set to 00 or 01 HAMC Tuple 21 bytes }

The handover indicator message (MOB-HO-IND) may include information tonotify the former serving BS 42 to resume communication with the MSS 41if the handover of the MSS 41 to the handover target BS fails.

Before the connection to the MSS 41 to the serving BS 42 is released,the MSS 41 may preferably execute a fast ranging (e.g., FastRanging_IE), a ranging request (e.g., RNG-REQ), and a ranging response(e.g., RNG-RSP) with the target BS selected for the handover (S419,S420, S421). In the embodiment, the target BS 44 is selected for thehandover. The ranging request may preferably require uplinksynchronization between the MSS 41 and the target BS 44. The fastranging may preferably comprise a frame slot allocation by the targetbase station 44 to enable the MSS 41 to use the allotted frame slot forperformance of fast ranging.

In one embodiment, if the handover is successfully performed, thehandover target BS 44 may transmit a handover result to the serving BS42 via a handover complete message (e.g., HO-Complete). The formerserving BS 42 may preferably recognize the successful completion of thehandover via the handover complete message and may release the servingBS's 42 connection to the MSS 41 (S423). Execution of initial networkentry may then be completed (S424).

An exemplary configuration of the handover complete message is shown inTable 2.

TABLE 2 Field Size Notes Global Header 152 bits MSS unique  48 bits48-bit unique identifier used by MSS on initial identifier network entryReason  8 bits #0: Successful Handover #1: Handover Failure due to xxx#2: Handover failure due to yyy #3~7: reserved Security field TBD Ameans to authenticate this message

The handover complete message may preferably be an inter-base stationbackbone message to provide notification of a handover state of the MSS41 from a new serving BS to the former serving BS 42. Using the handovercomplete message, the former serving BS 42 may determine whether the MSS41 has completed the handover to the handover target BS 44. In a casewhere there is a handover failure, the reason for the handover failure(failure reason) may preferably be set to be delivered to the formerserving BS 42. The failure reason may be, for example, a call drop thatcauses the MSS 41 to fail in accessing the handover target BS 44.

In the event that the handover process is completed, the former servingBS 42 may maintain resources and information associated with the MSS 41for a predetermined time duration (where there is no instruction fromthe new serving BS 44).

FIG. 5 is a diagram illustrating a handover process initiated by a BS,according to one embodiment of the present invention.

Referring to FIG. 5, a serving BS 52 triggers a procedure (e.g.,trigger-to-release-MSS) to release a connection from an MSS 51 accordingto a corresponding determination (S510).

The serving BS 52 may transmit a handover pre-notification message(e.g., HO-pre-notification) including information required for thehandover of the MSS 51 to neighboring handover target BSs 53 and 54(S511, S512). The handover pre-notification message may include anidentifier (MSS identifier) of the MSS requesting the handover,connection parameters, capabilities, a required bandwidth, and qualityof service information.

Each of the neighboring handover target BSs 53 and 54 may receive thehandover pre-notification message and may transmit a handoverpre-notification response message (e.g., HO-pre-notification-response)to the serving BS 52 in response to the handover pre-notificationmessage (S513, S514). The handover pre-notification response message maypreferably include acknowledgement information pertaining to thehandover pre-notification message and capability information pertainingto the requested quality of service.

The serving BS 52 may preferably transmit a handover response message tothe MSS 51 based on the information received via the handoverpre-notification response message (S515). The handover response messagemay preferably include information pertaining to the target BSs 53 and54 available for the handover. The MSS 51 may preferably perform aneighboring BS scanning procedure to measure quality of received signalson the neighboring target BSs, recognized via the handover responsemessage (S516).

In another embodiment, in a case where the neighboring BS scanningprocedure finds the target BS 54 for the handover, the MSS 51 maytransmit a handover response message including target BS information forthe handover to the serving BS 52 (S517). The serving BS 52 also maytransmit a handover confirm message to the target BS 54, with which theMSS 51 determines to perform the handover (S518).

The MSS 51 may transmit a handover indicator message (e.g., MOB-HO-IND)to release a connection from the MSS 51 to the serving BS 52 (S519). Thehandover indicator message may preferably be configured as shown inTable 1.

Before the connection of the MSS 51 to the serving BS 52 is released,the MSS 51 may preferably execute a fast ranging (e.g., FastRanging_IE), a ranging request (e.g., RNG-REQ), and a ranging response(e.g., RNG-RSP) with the serving target BS 54 for the handover (S520,S521, S522).

In yet another embodiment, if the handover is successfully performed,the handover target BS 54 may transmit a handover result to the servingBS 52 via a handover complete message (e.g., HO-Complete). The handovercomplete message may preferably be configured as shown in Table 2. Theformer serving BS 52 may preferably recognize successful completion ofthe handover via the handover complete message and may release itsconnection to the MSS 51 (S524). Execution of initial network entry maythen be completed (S525).

FIG. 6 is a diagram illustrating a process by which an MSS resumescommunications with a former serving BS, in case of a handover failure,according to one embodiment the present invention. The handover failuremay be due to, for example, a call drop or other failure reason.

Referring to FIG. 6, an MSS 61 performs a scanning procedure to measurequalities of signals transmitted from neighboring BSs (S10). If the MSS61 determines that a handover needs to be performed based on results ofthe neighboring BS scanning procedure, the MSS 61 may preferablytransmit a handover request message (e.g., MOB-MSS-HO-REQ) to a servingBS 62 (S611).

The serving BS 62 receives the handover request message and may transmita handover pre-notification message (e.g., HO-pre-notification)including information that the MSS 61 has requested a handover to aneighboring BS, such as for example, handover target BSs 63 and 64(S612, S613). The handover pre-notification message may also include anidentifier (MSS identifier) of the MSS requesting the handover,connection parameters, capabilities, a required bandwidth, and qualityof service information.

Each of the neighboring handover target BSs 63 and 64 may receive thehandover pre-notification message and may transmit a handoverpre-notification response message (e.g., HO-pre-notification-response)to the serving BS 62 in response to the handover pre-notificationmessage (S614, S615). The handover pre-notification response message maypreferably include acknowledgement information pertaining to thehandover pre-notification message and capability information pertainingto the requested quality of service.

The serving BS 62 may preferably select one of the neighboring handovertarget BSs 63 and 64 based on the handover pre-notification responsemessage and may transmit a handover confirm message (e.g., HO-confirm)to the selected target BS 63 or 64 (S616). The serving BS 62 may alsotransmit a handover response message (e.g., MOB-BSHO-RSP), includinginformation pertaining to the handover target BS, to the MSS 61 (S617).The MSS 61 receives the handover response message and may transmit ahandover indicator message (e.g., MOB-HO-IND) to release a connectionfrom the MSS 61 to the serving BS 62 (S618).

In still another embodiment, when the MSS 61 attempts to connect to thehandover target BS 64, a handover failure, such as for example a calldrop, occurs (S619). The MSS 61 may preferably recognize the handoverfailure and may notify the handover target BS 64. The MSS 61 may alsocancel the handover from the former serving BS 62 and may transmit amessage to resume communications with the former serving BS 62. Becausethe connection between the serving BS 62 and the MSS 61 was not yetreleased, the MSS 61 may maintain the connection with the serving BS 62without executing a procedure for re-entering the serving BS 62. Thus,although the handover attempt failed, the MSS 61 may maintain theconnection with the serving BS 62 without executing a procedure forre-entering the serving BS 62. In order to resume communications withthe former serving BS 62, the exemplary handover indicator message shownin Table 1 may preferably be used.

In one embodiment, a method of performing handover operation in awireless communication system includes communicating with a serving basestation through a first communication link and receiving from theserving base station, handover information associated with at least onecandidate target base station, wherein the handover informationcomprises at least quality of service capable of being handled by the atleast one candidate target base station. The method also includestransmitting a handover indicator to the serving base station indicatinga handover operation to a selected target base station, wherein themobile subscriber station determines whether to perform handover inresponse to the handover information, and performing a ranging procedurewith the selected target base station to establish a secondcommunication link with the selected target base station whilemaintaining the first communication link with the serving base station,wherein the ranging procedure comprises attempting to establish anuplink synchronization and power adjustment between the mobilesubscriber station and the selected target base station. The method alsoincludes being released from the serving base station in response to ahandover complete status message from the selected target base stationto the serving base station, or resuming normal communication with theserving base station if the handover fails with the selected target basestation.

The method may also preferably include receiving a handover responsefrom a serving base station upon the serving base station transmitting ahandover confirmation to a handover target base station, wherein thehandover response comprises at least quality of service informationrequested to be handled by the handover target base station. The methodmay also preferably include, in a mobile subscriber station initiatedhandover, transmitting a handover request to a serving base stationthrough a first communication link prior to receiving the handoverresponse from the serving base station. The method may also preferablyinclude, in a service base station initiated handover, receiving ahandover request from the serving base station prior to transmitting ahandover response to the serving base station.

The mobile subscriber station may preferably be released from theserving base station when the handover complete status message indicatessuccessful handover. The mobile subscriber station may preferably resumenormal communication with the serving base station when the handovercomplete status message indicates handover failure.

The step of performing the ranging procedure may preferably includereceiving a frame slot allocation from the target base station for afast ranging, transmitting a ranging request to the target base stationusing the allotted frame slot, and receiving a ranging response from thetarget base station.

The handover complete status message from the target base station to theserving base station may preferably be an inter-base station backbonemessage. The handover indicator may preferably include one of (1)serving base station release, (2) handover cancel, and (3) handoverrejection.

In another embodiment, a method of performing handover operation in awireless communication system includes receiving a handover request froma mobile subscriber station through a first communication link,transmitting a handover pre-notification to a target base station, andtransmitting a handover response upon the serving base station receivinga handover confirmation from the target base station. The method alsoincludes receiving a handover indicator from the mobile subscriberstation indicating a handover operation to the target base station andperforming a setup procedure between the mobile subscriber station andthe target base station to establish a second communication link withthe target base station while maintaining the first communication linkwith the serving base station. The method also includes releasing themobile subscriber station or continuing normal communication with themobile subscriber station in response to a handover complete statusmessage from the target base station to the serving base station.

In yet another embodiment, a method of performing handover operation ina wireless communication system comprises transmitting a handoverpre-notification to a target base station. The method also comprisestransmitting a handover response upon the serving base station receivinga handover confirmation from the target base station and transmitting ahandover request to a mobile subscriber station that is in communicationwith the serving base station through a first communication link. Themethod also comprises receiving a handover response from the mobilesubscriber station indicating a handover operation to the target basestation. The method also comprises performing a setup procedure betweenthe mobile subscriber station and the target base station to establish asecond communication link with the target base station while maintainingthe first communication link with the serving base station. The methodalso comprises releasing the mobile subscriber station or continuingnormal communication with the mobile subscriber station in response to ahandover complete status message from the target base station to theserving base station.

The handover request may preferably comprise a possible handover targetbase station. The method may also preferably comprise receiving ahandover complete message from the target base station to the servingbase station.

Accordingly, the present invention may efficiently execute a handoverand/or resume communications in a wireless access system when a mobilesubscriber station (MSS) returns to a former serving base station (BS),in a case of a failed handover.

It will be apparent to those skilled in the art that variousmodifications and variations may be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1-25. (canceled)
 26. A method of performing handover operation in awireless communication system, the method comprising: performing ascanning procedure to measure qualities of signals transmitted fromneighboring base stations, wherein the scanning procedure comprisestransmitting a scan request message for requesting scanning interval tothe serving base station and receiving a scan response message inresponse to the scan request message to assign scanning interval forscanning the neighboring base stations; transmitting a handover requestmessage to the serving base station if determining that a handover needsto be performed, wherein the serving BS transmits a handoverpre-notification message to neighboring base stations; receiving ahandover response message including information pertaining to one ormore neighboring handover target base station from the serving BS;transmitting a handover indication message to release a connection tothe serving base station, wherein the handover indication messagecomprises a handover indication type which is set to releasing servingbase station and an identifier of the handover target base station;receiving a fast ranging information from a handover target basestation, wherein the fast ranging information comprises resources whichis allocated for ranging request; transmitting ranging request messageusing the resources allocated by the fast ranging information to thehandover target base station and receiving ranging response message fromthe handover target base station to establish an uplink synchronizationand power adjustment with the handover target base station; and beingreleased by the serving base station if a predetermined time durationpasses after the serving base station receives a handover indicationmessage.
 27. The method of claim 26, wherein said one or moreneighboring handover target base station whose information is includedin the handover response message is selected by the serving basestation.
 28. The method of claim 26, wherein the fast ranginginformation is Fast Ranging_IE.
 29. The method of claim 26, wherein thehandover pre-notification message includes information that the mobilesubscriber station has requested a handover as well as identifier of themobile subscriber station.
 30. The method of claim 26, furthercomprising transmitting another handover indication message to theserving base station if the handover to the target base station fails.31. The method of claim 26, further comprising resuming communicationwith the serving base station if the handover to the target base stationfails.
 32. The method of claim 26, wherein determining that a handoverneeds to be performed is based on results of the neighboring BS scanningprocedure.